Moving mirrors vs. moving fixtures?

I remember when mirrors were it, man!  When I /finally/ got a chance to 
actually use any kind of 'intelligent' fixture, they were MAC 500 (and 
later a couple of others.....can't remember the name now :( )

The head guy there complained that the mirrors could never hit the same 
spot.  What's the verdict, or is it all dependent on year, make, model, 
and revision?

Torrance